Organisms vary in sensitivity to toxic environmental chemicals, depending on genetic and other factors. We are working on a model system with yeast, Saccharomyces cerevisiae, to investigate genetic changes occurring in response to mutation and selection in the presence of contaminating metals. Twenty-four independent control populations were established. From these populations additional population sets were derived, each set grown on media containing metal at concentrations sufficiently high to interfere with growth. Gradually, the populations become adapted and growth improves. As growth improves we increase the concentration of metal in the medium. Populations are maintained separately to permit analysis of alternative adaptive strategies and the different molecular mechanisms that might be involved. Strains that vary in their ability to grow in the presence of lead, mercury, nickel and cobalt have been isolated. A rapid method for comparing growth rates of many cultures simultaneously was developed. Using this method, we found that the majority of 24 Pb-adapted strains can be readily distinguished from all 24 nonadapted control strains when tested on Pb-containing medium. Also, metal-adapted strains appear to have a growth disadvantage on normal media compared to control strains. One Pb-adapted strain was found to be very highly resistant to mercury, and one Ni-adapted strain was highly resistant to cobalt. Several Pb-adapted strains became morphologically distinguished by a dark coloration, presumably due to the presence of some form of lead in the cells. Microscopic analysis reveals the color to be associated with cell structures. Chemical analysis showed considerable variation in Pb concentration between strains, suggesting that some adapted strains remove lead from the medium more efficiently than others. Two-dimensional gel electrophoresis revealed differences to distinguish control and Pb-adapted strains. Protein spots unique to control as well as Pb-adapted strains were observed. Many of the metal-adapted and control strains are polymorphic in terms of colony morphology when cells are plated on agar. Variation includes differences in color, size and shape. In some cases there are differences in the proportions of the different forms depending on presence or absence of metal in the medium. A small number of isolated mercury-resistant colonies recently appeared unexpectedly, suggestive of "selection-induced" mutation.